In order to store or transmit digital image efficiently, the image has to be compression-coded. Examples of efficient methods of compressing digital image include: JPEG for still pictures; and H.261, H.263, Moving Picture Experts Group (MPEG)-1, MPEG-2, and MPEG-4 for moving pictures. MPEG-4 AVC (Advanced Video Coding), standardized by ISO (International Organization for Standardization) in 2003, aims to improve the performances of MPEG-1 to 4 as well as sharing their technologies.
In the image data coding methods for a moving picture, the following two types of prediction modes are provided: intra picture prediction and inter picture prediction. In MPEG-4 AVC, for example, divided blocks (macroblocks) arranged consecutively within a picture, fall into two groups depending on coding method difference: intra macroblocks and inter macroblocks (see Non-Patent Document 1).
Intra picture prediction is a mode in which a prediction image (intra-prediction image) corresponding to a current block to be coded is generated based on the pixel information on their peripheral blocks within a closed picture so that the difference between the prediction image and its original image is transmitted. Inter picture prediction is a mode in which either or both of the pictures positioned forward and backward of a current picture are specified as reference pictures so that a prediction image in a current block is generated from the reference picture in order to transmit the difference between the prediction image and its original image.
FIG. 1 is a block diagram showing the showing the function and the structure of a conventional image decoding apparatus. As shown in this figure, the conventional image decoding apparatus, which decodes image coded signals each of which is generated by dividing an image into one or more blocks and then coding the block, in order to decode an intra-prediction image, includes a variable-length decode/de-quantization unit 101, an intra-prediction mode extraction unit 102, an intra-prediction image generation unit 103, a line memory 104, and an adder 105.
The conventional image decoding apparatus also decodes an inter-prediction image. However, this function does not relate to a primary feature of the present invention, and its description is omitted in the figure to avoid any confusion.
In the image decoding apparatus having such a configuration, the coded signals, each of which has been generated by dividing an image into one or more blocks and then coding the block, is transformed to differential block image data in a space region by the variable-length decode/de-quantization unit 101, and then outputted to the adder 105.
The intra-prediction mode extraction unit 102 extracts intra-prediction mode information contained in coded signals block by block so as to output the information to the intra-prediction image generation unit 103. The intra-prediction image generation unit 103 obtains reference pixels of a necessary reference block from the line memory 104 according to the extracted intra prediction mode, generates an intra-prediction image block by block, and then outputs the image to the adder 105.
In the adder 105, the intra-prediction image outputted from the intra-prediction image generation unit 103, and the differential block image data outputted from the variable-length decode/de-quantization unit 101, are added together to generate a decoded image.
The decoded image, or equivalently the addition result, is outputted from an output terminal as decoded signals, as well as is stored in the line memory 104 as a reference block image for next intra-prediction image generation.
Hereinafter, a description is given for intra prediction mode, with MPEG-4 AVC Baseline profile taken as an example.
In the intra-prediction image generation method of MPEG-4 AVC luminance signals, the following prediction block units are provided: Intra 16×16 formed from a 16 by 16 matrix of pixel blocks; and Intra 4×4 formed from a 4 by 4 matrix of pixel blocks. Each of the units defines its own intra prediction mode.
FIG. 2 lists Intra 4×4 prediction modes available in MPEG-4 AVC. FIG. 3 shows reference pixels and prediction directions employed for the intra prediction modes shown in FIG. 2.
The reference block is a block containing pixels necessary for intra-prediction image generation in an intra prediction mode specified in a stream (image coded signal). The reference block has already been decoded. The reference block is left-, above-, or upper right/left-adjacent to a current block.
There is shown DC (average value) mode in FIG. 3, in which an intra-prediction image is generated based on the average value of the reference pixels contained in the above-adjacent and left-adjacent reference blocks. In the case where either of the above and left adjacent reference blocks is unavailable, only the pixels of the other available reference block are employed to generate an intra-prediction image. Furthermore, in the case where neither of the above- and left-adjacent reference blocks are available, an intra-prediction image is generated, so that every pixel of the intra-prediction image has a value of 128.
In order to determine whether or not an adjacent reference block is available, MPEG-4 AVC standards define the following three types of blocks unavailable as a reference block: (a) a block not contained in a picture; (b) a block belonging to a slice different from the slice to which a current block to be decoded belongs; and (c) a block inter-picture predicted in the case where an intra prediction-restriction flag in a stream restricts the reference to an inter picture-predicted block.    Non-Patent Document 1: ITU-T Rec.H264|ISO/IEC 14496-10 version 1 “Information technology-Coding of audio-visual objects—Part 10: Advanced video coding”